The present invention generally relates to high power waveguide systems, and more particularly to waveguide circulators having several terminals or ports so arranged that microwave energy entering one port is transmitted to the next adjacent port in a defined manner.
Microwave circulators are well-known non-reciprocal microwave devices used in a variety of applications, including isolating a microwave power source from a reflective microwave load. By inserting a circulator between the microwave source and microwave load, the source can be isolated from the load without absorbing a significant portion of the generated power. For example, most conventional circulators provide isolation with insertion loss from about 3.5 to 5 percent of generated power, leaving 95 to 96.5 percent of the available power for the intended application. Typically, a microwave power source will be either a self-excited magnetron oscillator or a klystron amplifier. The magnetron oscillator is a less costly source of microwave power and can be used in lower power applications. However, both the power and frequency output of the magnetron is influenced by the magnitude and phase of the power reflected from the load, and can be damaged if load reflections are too large. Klystrons, on the other hand, are less sensitive to load reflections, but nonetheless can experience damage if such reflections are excessive.
Two types of waveguide circulators have heretofore been used to protect microwave sources. One is a three port waveguide junction circulator which is typically used for lower power applications, and another is a four port (phase shift) circulator which is used for higher power applications. Generally, the four port phase shift circulator design provides greater isolation and higher power handling capability than three port circulators, however, four port circulators are more expensive and have more demanding physical requirements due to the additional port of the circulator. The three port circulator, provides a less costly and physically less demanding alternative to the four port circulator.
The most general form of a three port junction circulator consists of a symmetrical distribution of a non-reciprocal ferromagnetic material at the junction of three waveguide transmission lines. In a usual configuration, an H-Plane waveguide T forms a junction where two thin ferrite discs are supported on two short posts protruding from the opposite broadwalls of the waveguide junction. The ferrite disks are magnetized in a direction that is perpendicular to the plane of the disks by a static magnetic field produced by a C-shaped magnet surrounding the junction. The non-reciprocal ferrite disks produce a rotation in the microwave energy at the junction of the waveguide T.
Despite its cost advantages, the three port junction circulator has a number of disadvantages. These disadvantages include limited power handling capabilities. This limitation is largely due to the field strengths generated in the gap between the relatively small disks and the difficulty of cooling the disks which generate increasing amount of heat with increased power levels. Further disadvantages include limited frequency bandwidth and sensitivity to temperature changes which, among other things, can be affected by changes in the temperature of the water coolant passed through the circulator. To overcome these disadvantages, efforts have been made to improve the heat transfer characteristics of the ferrite disks and to reduce the ferrite heating and E field breakdown in the gap by adding additional disks to a bifurcating septum between the posts on which the ferrite disks are supported. However, despite these efforts, power handling capabilities of conventional three port circulators have had difficulty keeping up with the increased power availability from microwave sources. Nor have improvements in these three port junction circulators overcome the frequency bandwidth or temperature sensitivity limitations inherent in these devices.
The present invention provides a three port circulator design which provides the benefits of a four port circulatorxe2x80x94increased power handling capability and frequency bandwidth and decreased temperature sensitivityxe2x80x94while maintaining the advantages of a conventional three port circulator, namely, cost advantages and reduced physical requirements. The distributed three port circulator of the present invention is particularly adapted for high power microwave systems such as resonant cavity electron accelerators which require isolation to protect and stabilize the microwave power source .
Briefly, the present invention involves a distributed three port waveguide circulator which includes a waveguide coupler section which preferably is a 3 db (hybrid) waveguide coupler, and a stacked waveguide section having dual waveguide paths, at least one of which is loaded with a distributed non-reciprocal ferromagnetic material for producing a phase shift in the microwave power traveling in one waveguide path relative to power traveling in the other. In the preferred embodiment of the invention, the waveguide sizes in the waveguide coupler section are full height guides and the dual waveguide paths in the stacked waveguide section are reduced height guides which combine at a stacked output end of the guides at full height. A transformer section, suitably a stepped waveguide transformer, is provided to couple the full height guides of the waveguide coupler section to the reduced height guides of the stacked waveguide section.
More particularly, the waveguide coupler section is comprised of a first waveguide path having defined terminals A and B, a second waveguide path having defined terminals C and D, and an apertured common wall portion for dividing power introduced to any one of the terminals A, B, C or D between the first and second waveguide path. The apertured common wall portion produces a ninety degree phase shift in the power coupled from one to the other of the waveguide paths such that the divided microwave power delivered to the coupler output at terminals B and C are ninety degrees out of phase. The divided microwave power will enter the two waveguide paths of the stacked waveguide section with this same phase relationship.
In its preferred design, the stacked waveguide section is a bifurcated waveguide section comprised of a section of waveguide corresponding in cross-sectional size and shape to the first and second waveguide portions of the waveguide coupler section. This section of waveguide is bifurcated into two stacked, approximately one-half height waveguides to provide dual reduced height waveguide paths through the bifurcated section. The bifurcated waveguide section has a bifurcated input end for receiving and transmitting divided power from and to the coupler output of the waveguide coupler section, and a bifurcated output end for delivering combined power from the dual waveguide paths of the bifurcated guide. At least one, and preferably both, of the reduced height waveguides of the bifurcated guide are loaded with distributed strips of a non-reciprocal ferromagnetic material such as ferrite or garnet (sometimes herein referred to as simply xe2x80x9cferritexe2x80x9d) extending along at least a portion of its length such that, when the ferromagnetic strips are properly magnetized in the presence of transverse static magnetic field, they act to produce a phase shift in the microwave power traveling through one of the reduced height waveguides relative to the other reduced height waveguide. A magnetic circuit, such as an arrangement of permanent magnets and pole plates, is provided in association with the bifurcated waveguide section for magnetizing the distributed ferromagnetic material. The ferrite loaded portion of the bifurcated waveguide section is of sufficient length to permit differential phase shifting of the power received by the stacked guides at the bifurcated input end of the bifurcated waveguide section to arrive substantially in phase at the section""s bifurcated output end to thereby permit the combined power from the dual waveguide paths of the bifurcated guide to be delivered to a microwave load. Conversely, the non-reciprocal property of the ferrite loaded portion of the bifurcated waveguide section will cause power traveling through the bifurcated guide which is reflected from the microwave load to experience differential phase shifting that is the reverse of the differential phase shift experienced by power traveling in the opposite direction to the load. Thus, reflected power arrives at the output end of the waveguide coupler from the bifurcated waveguide section in an out-of-phase relationship that is the reverse of the out-of-phase relationship of power fed from the coupler output to the bifurcated guide. This, in turn, permits the splitting and sebsequent summation of the reflected power within the waveguide coupler in a manner that directs the reflected power to the desired waveguide path of the waveguide coupler.
In the preferred embodiment, the reduced height waveguides of the bifurcated waveguide section are rectangular waveguides separated by a common web plate fabricated of a magnetic material, such as steel, which is plated with a conductive material such as copper or silver. This web plate forms an inner waveguide broadwall opposite the ferromagnetic strips and is a part of the magnetic circuit for producing a static magnetic field in the reduced height guides.
In a further aspect of the invention, means for cooling the ferromagnetic strips attached to the outer broadwalls of the bifurcated waveguide section are provided in the form of cooling tubes extending along the outside surface of the outer broadwalls of the reduced height waveguides in a position that is close to the ferromagnetic strips on the reverse sides of the broadwalls.
In still a further aspect of the invention, a method is provided for isolating a microwave power source from a microwave load. The method includes the step of introducing power from the microwave source to a first waveguide path of a waveguide coupler which divides the power into two first and second waveguide paths such that the divided power is approximately ninety degrees out-of-phase. The waveguide coupler also causes reflected power arriving at the coupler output to cancel in the first waveguide path and add in the second waveguide path. A further step of the method involves passing the divided out-of-phase power from the coupler output through stacked downstream waveguide paths having a stacked waveguide output. At least one and preferably both of the stacked waveguide paths are loaded with a lengthwise distributed non-reciprocal ferromagnetic material which causes the power traveling down one of the stacked waveguide paths to be phase shifted approximately ninety degrees relative to the power in the other stacked waveguide path such that power in both waveguide paths arrive at the stacked waveguide output substantially in phase, whereupon the microwave power is delivered to a useful microwave device or load connected to the stacked waveguide output. The ferrite loading of the dual downstream waveguide paths further causes power reflected back through the dual waveguide paths to be differentially phase shifted in reverse such that it arrives at the coupler output in the proper out-of-phase relationship that permits reflected power divided and summed in the coupler section to cancel in the first waveguide path of the coupler but add in the second waveguide path. A microwave power source connected to a first port of the circulator associated with this the first path will be isolated from the microwave load connected to a second port of the circulator at the stacked waveguide output of the stacked downstream waveguide paths when a well matched microwave load for absorbing reflected power is connected to a third port of the circulator associated with the second microwave path of the waveguide coupler.
Due to the use of a distributed ferromagnetic material in the three port waveguide circulator and method of the present invention, the heated ferromagnetic material will be easier to cool and will be capable of handling higher power levels as compared to conventional three port junction circulators. The distributed ferromagnetic material also will provide for greater bandwidth capabilities and decreased temperature sensitivity. The foregoing objectives are additionally achieved in a waveguide circulator construction that is generally less costly than four port waveguide circulators conventionally used in high power applications. Other objects and advantages of the invention will be apparent from the following specification and claims.